Shelf unit



FIG. 1 is a perspective view of a shelf unit showing my design;

FIG. 2 is a front elevation;

FIG. 3 is a top plan view;

FIG. 4 is a left side elevation;

FIG. 5 is a right side elevation; and

FIG. 6 is a bottom plan view. 

The ornamental design for a shelf unit, substantially as shown. 